Here's a quick tutorial on getting reasonable star colours across a wide range of star intensities.
First download my test image: http://www.markshelley.co.uk/webdisk/star_colour_test_image.tif
In your favourite post-processing system, subtract the background light pollution, perform a white balance and then stretch. Do you get a result like this:
(http://www.markshelley.co.uk/webdisk/star_colour_iris_result.jpg)
If not, try this:
Download IRIS and use File|Load to open the TIFF file.
If you can't see the Threshold dialog then do a View|Threshold. Hit the Auto button. This doesn't change the image data - only the way it is displayed.
Open a command window by clicking the Command icon: (http://www.astrosurf.com/buil/us/iris/cmd.gif)
If you haven't managed to use the threshold dialog then type "visu 1960 0" into the command window. This doesn't change the image data - only the way it is displayed.
With the mouse, draw a rectangle in the background "sky" and type "black" into the command window. This removes the light pollution.
Type "rgbbalance 1.17 1 1.4" into the command window. Alternatively, draw a rectangle on something that is supposed to be white and type "white" into the command window.
Type "offset 20" into the command window. This creates a data pedestal of 20. A value of 5 or 10 would also work.
Adjust the displayed visual range by typing "visu 32767 0" into the command window by adjusting the Threshold dialog sliders. Again this doesn't change the image data.
Type "reduce_hdr1 1.8" into the command window to apply a gamma of 1.8 - play around with different numbers if you wish depending on the effect you want to achieve in your own image data.
Then do File|Save as a TIFF file and re-load it into Photoshop or whatever you use next.
Mark
Thanks Mark will give this a try later, how do you subtract background LP? I just do a colour balance is that the same thing?
Carole
I gave that a try Mark thanks, but the trouble is these days I don't tend to have any DSLR images to practice on, I could try to see if I still have some old data.
Would it work on combined mono RGB images I wonder?
Carole
Quote from: Carole
I gave that a try Mark thanks, but the trouble is these days I don't tend to have any DSLR images to practice on, I could try to see if I still have some old data.
Would it work on combined mono RGB images I wonder?
Carole
Yes it would work with combined RGB images and with false colour narrowband as well. In either case save them as a 16bit TIFF.
Mark
I found an old HH DSLR image.
All goes well until I type in "reduce_hdr1 1.8" to the command window, at which point the software stops responding for a short while and then when it comes back to life it has completely demolished the image and all I am left with is a black image.
Carole
Quote from: Carole
I found an old HH DSLR image.
All goes well until I type in "reduce_hdr1 1.8" to the command window, at which point the software stops responding for a short while and then when it comes back to life it has completely demolished the image and all I am left with is a black image.
Carole
Try scaling the data before you start so pixel values are in the (approximate) range 0-32767